When we are recording a simulation it always defaults to a pearl theme. Is there a way to change this so that it defaults to a white theme while recording?
It is not because it is a software simulation, it is just because Pearl has been set as default theme.
You can always set another theme as default theme. Open the Themes dialog box from the big button Theme, right-click the theme you want to become default theme, and choose the option.
For a software simulation it doesn't really matter that much, because it will always be the Blank master slide which is used. The style for the messages is set up in the Preferences, Recording, Defaults and not in the Object Style Manager. Contrary to the global defaults for text containers, for those messages the old Captions are used instead of shapes. But you can change that also under Preferences, Recording, Modes to shapes.
As you probably know, you can set Global Preferences. If you do that they will be applied to a new software simulation as well. This means that another default Theme is not so relevant. You can always change the theme later on when you want to add content slides and/or change fonts.
Maybe this post could clarify more: